Increasing cost of fossil fuel and environmental concerns have stimulated worldwide interest in developing alternatives to petroleum-based fuels, chemicals, and other products. Biomass (material derived from living or recently living biological materials) is a possible renewable alternative to petroleum-based fuels and chemicals.
Plant biomass is the most abundant source of carbohydrate in the world due to the lignocellulosic materials in its cell walls, making it a desirable source of biomass. Lignocellulosic biomass includes three major components: (1) cellulose, a primary sugar source for bioconversion processes, includes high molecular weight polymers formed of tightly linked glucose monomers; (2) hemicellulose, a secondary sugar source, includes shorter polymers formed of various sugars (e.g., xylose, mannose, glucose, galactose, etc.); and (3) lignin that includes phenylpropanoic acid moieties polymerized in a complex three dimensional structure.
Plant cell walls include up to three layers, the two most common being primary cell walls and secondary cell walls. The primary cell wall provides structure for expanding cells and is composed of major polysaccharides (cellulose, hemicellulose, and pectin) and structural proteins (i.e., glycoproteins). The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network that is embedded in the pectin matrix. The outer part of the primary cell wall is usually impregnated with cutin (e.g., omega hydroxyl acids and their derivatives) and wax, forming a permeability barrier known as the plant cuticle.
The secondary cell wall, which is produced after the cell has finished growing, contains a wide range of additional compounds including polysaccharides and lignin. The lignin interpenetrates the cellulose, hemicellulose and pectin of the primary cell wall to provide additional strength via covalent cross-linking with the hemicellulose. While the relative composition of polysaccharides varies between plants, cell type, and age, the composition of lignocellulosic biomass is roughly 40-50% cellulose, 20-25% hemicellulose, and 25-35% lignin, by weight percent.
The additional compounds, or minor components, present in both the primary and secondary walls include a variety of species (e.g., inorganic materials, color bodies, and waxes) at varying concentrations. Additional minor components may derive from material associated with the production, processing, or handling of lignocellulosic biomass, such as soil or fertilizer. The minor components can be divided into two categories: (1) extractives, non-structural biomass components including terpenoids, stilbenes, flavonoids, phenolics, aliphatics, lignans, alkanes, and proteinaceous materials; and (2) ash, inorganic components such as aluminum, barium, calcium, iron, potassium, magnesium, manganese, phosphorous, sulfur, chloride, ammonium, sulfate, sulfite, thiol, silica, copper, carbonate, phosphorous, etc.
Very few cost-effective processes exist for efficiently deconstructing biomass and converting cellulose, hemicellulose, and lignin to components better suited for producing fuels, chemicals, and other products. This is generally because each of cellulose, hemicellulose, and lignin demands distinct processing conditions, such as temperatures, pressures, solvents, catalysts, reaction times, etc., in order to effectively break apart their polymeric structures. As a result, most processes are effective for converting only specific fractions, such as cellulose and hemicellulose, leaving the remaining fraction(s) behind for additional processing, or alternative uses.
The deconstruction process can introduce new compounds into the feedstock from the degradation of the biomass components. For example, deconstruction of biomass results in the deconstruction of polysaccharides into more desirable smaller saccharides, for example mono-, di-, or trisaccharides. The deconstruction process also introduces degradation products into the feedstock. The presence of sugar degradation products like organic acids and cyclic ethers signifies a lowering of the overall yield of desirable saccharides. As a result, one would expect the presence of sugar degradation products to be undesirable, and their production should be minimized. Surprisingly, the present methods are not only tolerant of sugar degradation products, but the sugar degradation products improve the effectiveness of the process to produce desirable monooxygenates from the biomass-derived feedstock
Regardless of the deconstruction process used, the resulting feedstock is likely to contain the desired oxygenated hydrocarbons (e.g. sugars) as well as sugar degradation products, extractives, ash, mineral salts, mineral acids, and other solvents used in the deconstruction. The latter components in the heterogeneous mixture can have an impact on biomass conversion efficiencies. Ash components, even at relatively low concentrations, can severely limit thermochemical, biochemical, and catalytic conversion of biomass by affecting operating temperatures, inhibiting fermentation, and poisoning catalysts. As a result, methods for purifying biomass-derived feedstocks prior to conversion and processes that are semi-tolerant to extractives and ash components are of interest. The latter could be especially important in making biomass a realistic alternative to petroleum feedstocks, as highly or completely pure feedstocks could carry additional costs, such as capital expenditures on equipment and processing systems.